1. The Field of the Invention
The invention relates to virtual tape systems and more particularly to the creation of multiple physical volumes in a manner that is transparent to the host of the virtual tape system.
2. The Relevant Art
High density, removable media storage libraries are used to provide large quantities of storage in computer systems. Typically, such data storage systems are employed for backup or other types of secondary storage purposes, but may be used as primary storage in circumstances that are conducive to sequential data access and the like.
The data is stored on media cartridges, such as magnetic or optical disks, that are arranged in storage bins and accessed when data on a cartridge is requested. Currently available media cartridges are capable of storing much more data than early media cartridges. For example, a data volume that corresponds to a 400 Megabyte disk may now be stored on a disk with up to 60 Gigabytes of storage capacity. Unfortunately, much legacy equipment in existing computer systems is configured for the smaller data volume sizes.
Volume mapping is used to create a correlation between the physical capacity of a storage cartridge (stack volume or physical volume) and the data storage unit size (virtual volumes or logical volumes) of a file or block that is stored on the cartridge. Given the available data storage capacity of a disk, such mapping allows multiple logical volumes to be stored on a single physical volume, hence providing an efficient use of the available storage media. A virtual tape server (VTS) is one device capable of creating and maintaining such mapping among physical volumes and logical volumes.
In certain circumstances, it may be desirable to make two physical back-up copies of a single logical volume. For example, if the data is particularly important, it may be wise to store it in multiple locations to avoid loss due to failure of a single tape or tape drive unit. Systems and methods have been proposed to create two physical copies of a logical volume in a virtual tape system. Unfortunately, a number of deficiencies exist in such known systems and methods.
For example, many known dual copy systems require an explicit command from the host to initiate creation of a second copy. The software running on the host must be modified to provide the dual copy command. As a result, such a dual copy system may be difficult to use with existing (legacy) software. Consequently, adding dual copy capability to an existing system may be rather difficult. Such dual copy systems may also require that the host transmit the data to the VTS again to create the second copy. Thus, the I/O resources of the host are unduly taxed.
Furthermore, some dual copy systems require the host to track the locations, i.e., the physical storage sites, of the logical volumes. This may require the maintenance of a database on the host to store metadata for each file stored in the VTS. Again, such dual copy systems are difficult or impossible to incorporate into existing host systems without providing new host software, and may unduly tax the resources of the host. Additionally, such a system may be difficult to use in a heterogeneous environment, i.e., with host computers that use different operating systems, file formats, etc.
Additionally, many known dual copy systems are somewhat inefficient in their use of VTS resources. In some instances, the primary copy is made from the virtual volume, and then the secondary copy is made from the primary copy. This requires extra time due to the need to mount and read from the tape on which the primary copy is stored. Additionally, VTS memory is required to transport the data from the primary location to the secondary location.
Thus, it would be an advancement in the art to provide a virtual tape system capable of making multiple copies of a logical volume in a manner that is substantially transparent to the host and thus does not burden the host's resources. It would be a further advancement in the art to provide a virtual tape system capable of efficiently storing and accessing file location information independent of the host. Yet further, it would be an advancement in the art to provide a virtual tape system that minimizes the VTS resources required to make the additional physical copies.